The present invention relates generally to a pressure sensor and, more specifically, to a fiber optic pressure sensor having a relatively small diameter so that the pressure sensor is suitable for insertion into the human body particularly in patients such as children having narrow arteries.
After open heart surgery, the blood pressure of the patient must be monitored for several days, typically 72 hours, after surgery. It is necessary to monitor both the diastolic and systolic blood pressure inside the patient's heart chambers to monitor for possible clogging of the arteries. After brain surgery, continuous monitoring of brain fluid pressure is also required. For children in particular, monitoring fluid pressures in the brain and in their arteries may be impossible using conventional pressure sensors since conventional pressure sensors have a relatively large diameter and would constrict the flow of the fluid being measured.
Electronic pressure catheters exist that may be used to measure the fluid pressures in a human body. For example, Millar's 2 French diameter unit which has a relatively small diameter is very expensive. Other less expensive electronic catheters such as Millar's 9 French is 3 mm in diameter.
Other devices employ remote measuring in which pressure tubing and flush devices are required. Remote measurement requires the high cost of surgery and discomfort to the patient. Other known pressure sensors include fiber optic devices. Fiber optics have the ability to bend easily and conform to the blood vessels while they can be manufactured out of bio-compatible materials. Known fiber optic pressure catheters use small moving mechanical parts. Commonly, the mechanical part is a optically reflective membrane. The membrane forms a mirror with respect to the light emitted from the end of the optical fiber. The membrane is stressed and deformed as a function of the pressure differential applied to it. As the deformation of the membrane continues, the amount of light reflected back into the fiber is a function of the applied pressure.
One problem with such sensors however, is that the membranes must be precision mounted in the sensor head. Because these membranes are on the order of one micron thick with a diameter of about 700 microns, it is very difficult to mount them in the sensor head without introducing significant mechanical hysteresis and without creating a tendency to buckle. Although some of the hysteresis effect may be corrected by software, production yield is very low. Therefore, such units are restrictively high in price which limit its applications. Even so, the smallest known sensor has a relatively large diameter and cannot be used for children and infants.